Programming Fundamentals › Programming Basics

Short-Circuit Evaluation

&& and || stopping as soon as the result is known.

Also known as: short circuit, lazy evaluation of boolean operators, and or short-circuit

Short-circuit evaluation means that and / && and or / || stop evaluating as soon as the result is known. The right-hand side may never run.

False and expensive()     # expensive() is never called: the result must be False
True or expensive()       # never called: the result must be True

A useful guard

Because the right side is skipped, you can safely check something before using it:

if user and user.is_active:        # user.is_active only runs if user isn't None
    ...

if items and items[0] == "x":      # no IndexError on an empty list
    ...

const city = user && user.address && user.address.city;   // the old way; today use user?.address?.city

Providing defaults

or / || returns the first truthy value, and and / && returns the first falsy one, not just True/False:

name = given_name or "Anonymous"

Careful: 0, "" and [] are falsy, so count or 10 replaces a real 0. In JavaScript, use ?? to replace only null and undefined (truthy and falsy).

Pitfalls

• Side effects on the right can be skipped. ok = check_a() and save() won’t call save() if check_a() fails. That’s usually the point, but don’t rely on it for hidden behavior.
• Order matters. Put the cheap or safe check first.
• The & and | operators don’t short-circuit. They’re bitwise (bitwise operations).